IMPORTANCE OF PHYSICAL, CHEMICAL AND BIOLOGICAL PROCESSES IN THE DEVELOPMENT OF TOXIC GYMNODINIUM CATENATUM BLOOMS IN SOUTH-EAST TASMANIA, AUSTRALIA

Water column stratification is one of the primary factors involved in toxic G. catenatum bloom development in Tasmania's (Australia) Huon Estuary. Other potential factors influencing bloom initiation are (i) influx of humic substances and other micro-nutrients (e.g. selenite) in riverine flow facilitating growth of vegetative cells; and (ii) strong winds which result in sediment resuspension and resting cyst germination. The most extensive G. catenatum blooms have developed during prolonged calm weather, however blooms can also develop and persist under varied conditions of temperature and wind-driven turbulence. A critical factor in 1997/8 summer bloom decline was the increasing occurrence of strong SE winds and influx of colder oceanic water (containing fast growing diatoms), resulting in breakdown of water column stratification, decreased G. catenatum growth, increased competition, and horizontal advection of G. catenatum cells out of the Estuary. Production of benthic resting cysts during sexual reproduction and grazing by the heterotrophic dinoflagellate Polykrikos schwartzii, tintinnids, and copepods may also play a role in bloom decline. Physical, chemical, and biological processes therefore all regulate G. catenatum blooms. Ongoing research focuses on building a hydrodynamic and ecosystem model of the Estuary, with the ultimate aim of decreasing the risk of such PSP events.